The present invention relates to a method of shape control for a hot or cold tandem mill for rolling strips.
Methods of shape control for controlling the shape of product, particularly control of long-edge shape or long-middle shape (these shapes are shown in FIG. 1, the former by (a) and the latter by (b)) of the product of a strip mill, are known in the art, in which the rolls of a single stand in the mill are forcibly deformed externally by means of a roll bender or alternately the corrections of product shape are effected through control of tension by tension control means.
For example, the use of tension control means for this purpose is well known from pages 1214 to 1222 of "Stahl und Eisen" No. 90, Vol. 22. However, the application of control by such known shape control means is invariably confined to a single stand, and there is no known method of shape control which takes into account the relationship between the individual stands of a series of rolling stands constituting a tandem mill.
It is common knowledge that in a so-called tandem mill comprising a series of rolling stands, there exists interstand tension which acts on the material to be rolled. Therefore, if any of the known methods of shape corrrection are applied as such to a tandem mill, variations in the interstand tension between the stand selected for control and the preceding stand, as well as the interstand tension between this selected stand and the succeeding stand, result in variation in the rolling loads of the three stands, with resultant variations in the thickness and shape of the rolled product. In other words, this application of the known methods would give rise to unnecessary changes in the shape that would be added to the desired shape corrections effected at the delivery side of the respective stands, and thus the desired shape correcting effect would not be obtained. Secondly, if the selected stand is not the final stand, the shape correcting effect obtained at the selected stand will be successively reduced during the rolling at the succeeding stands. Thus, the desire shape control effect cannot be achieved at the delivery side of the final stand. Thirdly, and lastly, the change in the strip thickness due to the shape correcting action will bring into operation the automatic gauge control (AGC) system, which further changes the shape of the strip. Thus, there is the danger of not only interfering with the desired shape correcting action, but also causing the entire control system to hunt. For these reasons, it is apparent that the primary object of shape control cannot be achieved.
In other words, since the aforesaid known methods of shape control are designed for use on a single stand, these known methods are wholly inapplicable to tandem mills. There thus exists the need for a method of shape control for tandem mills, which takes into consideration the mutual effects of the stands and the relationship with other control systems such as the gauge control and the like.